1. Field of the Invention
The present invention relates to a focus control apparatus provided in a reproduction apparatus which reproduces data from a multilayer optical recording medium.
2. Description of Related Art
One way to improve the recording density of a disk is multiplexing of information in a direction perpendicular to the disk's surface. A multilayer optical disk is capable of implementing such perpendicularly multiplexed information recording. With a double-layer optical disk in use, for example, first and second layers are formed with a spacer region in between as shown in FIG. 1, and the first layer closer to the light irradiated surface of the disk is made translucent so that light reaches the second layer.
In reproduction of such a multilayer optical disk, changing the layer from which recorded information is to be read requires that a focus control apparatus should carry out a focus jump operation to rapidly focus read light on the next reading layer.
Normally, the focus jump operation to shift the focal point of read light from the information recorded surface on one layer to the information recorded surface on another is executed based on zero-crossing detection of a focus error signal which is generated based on the output of a pickup.
Specifically, the pickup is provided with a focus actuator, a read-light emitting optical system, for example, which moves an objective lens for determining the focal point of the optical system along the optical axis to thereby shift the focal point of the read light in a direction perpendicular to the surface of the disk. In the initial stage of the focus jump operation, this focus actuator is supplied with an acceleration signal or a focus jump enable signal to shift the focal point of the read light to a target recording surface. Based on the timing of zero-crossing which is detected one after another from a focus error signal acquired during displacement of the focus actuator that is implemented in response to the acceleration signal, a series of operations of terminating the supply of the acceleration signal, supplying a deceleration signal to the focus actuator to stop the displacement of the focus actuator that has taken place in response to the acceleration signal or stopping supplying the deceleration signal and restarting focus servo on the target recording surface.
The focus actuator of a focus control apparatus, like that in a reproduction apparatus for ordinary optical disks, is affected by the ambient temperature. In a focus jump operation conducted by the above-described focus control apparatus for a multilayer optical disk, therefore, the focus position of read light may not be shifted properly to the recording surface of a target layer due to the influence of the ambient temperature on the focus actuator. When the ambient temperature is high, for example, the moving distance of the focus actuator becomes too short for the focus position to reach the target layer even with the use of the same acceleration signal and deceleration signal as generated at the normal temperature. When the ambient temperature is low, on the other hand, even the same acceleration signal and deceleration signal as generated at the normal temperature increase the amount of overshooting of the focal point of read light from the recording surface of the target layer so that the focus position jumps over the target layer.